I've been thinking about coin cell operated Moteinos for a bit and looked more into power consumption during sleep. As it turns out the most costly subsystem during sleep is the watchdog timer consuming a hefty 5 uA. If you build a sensor using energy efficient components (say the si7021) it's clear that the WDT is the main energy drain even if measurement and radio are figured in.
It occurred to me that I can shave off part of that by using listen mode to trigger measurements and switching the WDT off during sleep. The Moteino will be woken from sleep by the radio triggering an interrupt.
So I'll have one Moteinos broadcast 3s bursts every 10 minutes to wake up all thermometers and they will report back with the temperature. Sleep power should go down to about 3uA total with that approach. Add about 2uA for sending the update or an average total of 5uA which gives a couple of years on a cr2032.
Funny you should mention this. I'm in the middle of testing a similar setup and here's what I've found:
The average current while sleeping and an SI7021 (and an idle BMP180 since I'm prototyping with a Weathershield) is 4.3uA with a coin cell. The average calculated current with RFM69W transmitting every 10 minutes at about mid power is about 5.2uA. All good and, as you say, the coin cell should last for years.
The problem is startup. Loading the sketch and initially charging the bulk caps takes a LOT out of the coin cell and, in fact, if you're not careful, the coin cell discharges so much in this initial startup it can never get to sleeping mode. My plan of attack is to install the coin cell with a well established procedure that avoids the initial shock to the battery.
The procedure is to program the module with the sketch using separate power supply (no battery installed) and do all configuration (set node id, establish connection to gateway, etc) at that point. Then, to install the battery, use a power source to operate the node (and precharge the bulk caps), let the node sleep, install the battery, and disconnect the external power source.
This procedure SHOULD allow the device to operate for years on the coin cell. Unfortunately, the node is totally intolerant of any 'problems'. One hang, Gateway goes out and the node retries multiple sends, anything, and your dead, literally. The coin cell just does not have the capacity to operate an actively listening or transmitting node for more than a few seconds before the voltage collapses to an unusable (and unrecoverable) voltage.
My latest code doesn't use retry because it's too dangerous to the node. We'll see if that allows the node to live up to its expectations.
Tom